Polyolefins are conventionally stabilized with a phenolic antioxidant and a phosphite. A hindered amine light stabilizer (or “HALS”) may also be included if long term stabilization is desired. These formulations generally provide adequate stabilization performance during the melt processing operations which are used to produce finished plastic parts from the thermoplastic polyolefin resin and during the normal lifetime of the parts.
A general overview of phenol/phosphite stabilizer systems is provided in Polyolefins 2001—The International Conference on Polyolefins, “Impact of Stabilization Additives on the Controlled Degradation of Polypropylene”, p. 521, presented by Urs Stadler of Ciba Specialty Chemicals Corporation. For convenience, a summary of the teachings of this paper is provided below. It is generally accepted that:
1. Carbon centered radicals (or “CCRs”) are formed when polyolefins are subjected to heat and sheer during melt processing;
2. CCRs react with oxygen to form peroxy radicals (or “PRs”); and
3. PR's react with the polyolefin to form hydroperoxides (or “HPs”) and more CCRs.
In a conventional phenol/phosphite stabilizer system, it is generally accepted that:
1. the phenolic (which is also referred to as the “primary” antioxidant) decomposes the carbon centered radicals and peroxy radicals; and
2. the phosphite (which is also referred to as the “secondary” antioxidant) decomposes the hydroperoxides.
In addition, it is generally accepted that (a) the phenolic antioxidant provides a synergistic effect at the temperatures which are encountered during melt processing by reducing the rate at which the phosphite is depleted or consumed; and (b) if/when the phosphite is depleted (to below the concentration which is required for the decomposition of the hydroperoxides) then the structure of the polyolefin is altered by the action of the free radicals. In polypropylene, this generally results in “chain scission” (a reduction in the molecular weight of the polypropylene) whereas “crosslinking” is generally believed to predominate in polyethylene (which results in an increase in apparent molecular weight).
Stabilizer packages which comprise a combination of a phenolic antioxidant and a phosphite are in wide spread commercial use and generally provide efficient and cost effective results.
However, the use of phenolics is associated with color development in the finished plastic parts, especially in polyethylene or polypropylene which is produced with a transition metal catalyst (such as titanium, vanadium, zirconium or chromium). For example, “yellowing” and “pinking” have been observed in plastic parts which are stabilized with a phenolic antioxidant.
Thus, as noted in United States patent (“USP”) U.S. Pat. No. 5,596,033: “there is a long-felt need in the market place for a stabilizer system which can prevent color formation associated with the use of phenolic antioxidants”.
Several “phenol free” stabilization systems have been proposed and brief overviews of several of these systems follow:
1. Phosphites plus hindered amines (HALS): these phenol free systems are reported to lack adequate process stabilization (see U.S. Pat. No. 5,596,033).
2. N,N-dialkylhydroxylamine plus HALS: these phenol free systems are expensive (due to the high cost of the hydroxylamine) and may have restricted use in certain food contact applications due to FDA regulations (again, due to the presence of the hydroxylamine) (U.S. Pat. No. 5,596,033).
3. HALS, plus hydroxylamine (or nitrone) plus phosphite: as above, these phenol free systems are expensive and may have restrictions upon their use (U.S. Pat. No. 6,664,317).
4. HALS plus hydroxylamine plus a mixture of two hydrolytically resistant phosphites: as above, these phenol free systems are expensive and may have restrictions upon their use (U.S. Pat. No. 6,077,890).
5. Lactones (such as benzofuranone) plus HALS: these phenol free systems are also expensive (due to the high cost of the lactone) and may have restricted use due to FDA regulations concerning the use of lactones (U.S. Pat. No. 5,883,165).
We have now discovered an effective low cost, phenol free stabilization package for the melt processing of polyolefins.